In today's circuit switched telecommunications networks payload traffic and signaling are handled in different logical networks. Signaling traffic pertains to the signaling plane while the payload traffic is transported in the user plane.
The user plane of a call between two radio terminals, such as GSM terminals, is routed from a Base Station Subsystem (BSS) network to a core network and back to a BSS network. This procedure is performed irrespective of the location of a call, more precisely: irrespective of the locations of the involved mobile stations.
Hence, even when the call is “local”, i.e. both, the terminating and originating party, are served by the same Base Station Subsystem, e.g. within the same radio cell, the user plane is routed through the core network.
Currently it is under discussion in 3GPP standardization bodies to introduce “local switching” in case one single BTS (Base Transceiver Station) or one BTS-cluster or one single BSS (Base Station Subsystem is controlled by one Base Station Controller) is involved in the call, i.e. both radio terminals of a mobile-to-mobile call are served by the same radio access network.
It is proposed in 3GPP that the MSC signals towards the BSC for each call leg a kind of “call leg identifier”.
The BSS uses the call identifier(s) to correlate the two call legs. If the BSS received on both A-interfaces an indication that the controlling MSC(s) agree(s) to perform local switching and if the BSS identifies that the situation in the BSS allows establishment of local switching, then it performs a procedure to establish local switching. That means the user plane traffic between calling and called subscriber is switched locally inside the BSS network, bypassing the core network.